Species‐Specific Hydrolysis Kinetics of N‐Methylated Heroin Derivatives

The hydroxide‐catalyzed hydrolysis of 3,6‐diacetylmorphine (heroin) was shown to take place predominantly via its positively charged form. N‐Methylated quaternary derivatives of heroin bearing a permanent positive charge were synthesized, and thus, hydrolysis kinetics of these cationic species could be studied over a wide pH range. Specific rate equations were introduced to characterize either the simultaneous or the consecutive decompositions. The kinetic constants determined for the diester are distinctive for the site of hydrolysis. The rate of 6‐acetyl‐N‐methylmorphine was quantified in terms of microscopic kinetic constants of hydrolysis, in which the protonation state of the phenolic OH group had also been taken into account. The site‐specific data indicate that the 3‐acetoxy moiety is hydrolyzed 6 – 12 times faster than the 6‐acetoxy function. The latter, previously ignored minor pathway was shown to represent a non‐negligible 10% of the overall decomposition process. Protonation of the 3‐O⁻ site accelerates the rate of hydrolysis of the 6‐acetoxy moiety by a factor of 4, and replacement of the adjacent OH group by MeO or AcO substituents slows the rate of hydrolysis slightly, presumably due to the increased local hydrophobicity caused by the alkyl or acyl moiety.     

Mechanisms for the solvolytic decompositions of nucleoside analogues—VIII: Acidic Hydrolysis of 5-substituted 1-(alkoxyethyl)cytosines and cytidines

Several 5-substituted 1-(l-alkoxyethyl)cytosines have been prepared and the rate constants for their hydrolysis determined at various concentrations of oxonium ion. The acidity constants for the monoprotonated substrates and the rate constants for their decomposition have been calculated from the pH-rate profiles obtained. The effecs that varying the polar nature of the l-alkoxyethyl group exerts on the heterolysis of the monoprotonated substrates are interpreted to indicate that the acidic hydrolysis of l-(l-alkoxyethyl)cytosines proceeds by rate-limiting departure of the protonated base moiety with formation of an oxocarbenium ion intermediate. The same mechanism is extended to the hydrolysis of cytidines by comparing the influences that the 5-substituents have on the heterolysis of protonated 5-substituted l-(l-ethocyethyl)cytosines and correspondingly substituted cytidines.     

Analytical solutions to mathematical models of the surface and bulk erosion of solid polymers

The process by which polymeric materials hydrolyze and disappear into their environments is often called erosion. Two types of erosion have been defined according to how the hydrolysis takes place. If hydrolysis occurs throughout the entire specimen at the same time, it is called bulk erosion. If the hydrolysis is mainly confined to a region near the surface of the specimen and the surface continuously degrades by moving inward, it is termed surface erosion. In this article, a kinetic relationship for bulk erosion is developed. This relationship provides a method for estimating the hydrolysis kinetic constants for bulk‐eroding polymers. This same relationship is also applicable to surface erosion at a microscopic level. Through its combination with a diffusion–reaction equation and the provision of moving boundary conditions, an analytical solution to the steady‐state surface‐erosion problem is obtained. The erosion rate, erosion front width, and induction time can all be expressed as simple functions of the rate of polymer bond hydrolysis, water diffusivity, and solubility, plus other parameters that can be experimentally determined. The erosion front width is the product of the induction time and the erosion rate. The ratio of the erosion front width to the polymer specimen thickness is a parameter that determines whether the specimen undergoes surface or bulk erosion. Theoretical results are compared with experimental observations from the literature, and agreement is found. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 383–397, 2005     

Hydrolysis of 3,4-diphenyl-1,2,5-thiadiazole-1,1-dioxide. Acid and base catalysis

We have performed a product and kinetic study of the hydrolysis of 3,4-diphenyl-1,2,5-thiadiazole-1,1-dioxide in aqueous solution. Benzil and sulfamide are the only products of hydrolysis and are formed in equimolar yields. The kinetic results indicate that a first order law is followed up to 90% conversion. The observed rate constant is independent of substrate concentration. Ionic strength and buffer concentration do not affect the rate constant, but the reaction is acid-base catalyzed. The rate-pH profile has been determined and a mechanism that fits the experimental data satisfactorily is proposed. Corresponding rate constants and equilibrium constants for the protonation of the substrate are reported.     

Kinetics of alkaline hydrolysis of organic esters and amides in neutrally-buffered solution

Reaction rate constants for the hydrolysis of organic esters and amides were determined at temperatures of 100–240°C in aqueous solutions buffered at pH values between 5.5 and 7.3. Experiments are modeled assuming alkaline hydrolysis with a thermodynamic solution model included to account for the temperature dependence of hydroxide ion concentration. In most cases, the ester hydrolysis second order rate constants agree well with published values from experiments in strongly basic solutions at pH values from 11 to 14 and temperatures from 25–80°C, despite the large extrapolations required to compare the data sets. The amide hydrolysis rate constants are about one order of magnitude higher than the extrapolated results from other investigators, but the reaction rate increased proportionally with hydroxide ion concentration, suggesting that an alkaline hydrolysis mechanism is also appropriate. These data establish the validity of the alkaline hydrolysis mechanism and can be used to predict hydrolysis reaction rates in neutrally-buffered solutions such as many groundwater and geothermal fluids.     

13CNMR测定氨基甲酸钾盐的(月先)胺键水解平衡常数

本文用~(13)C NMR测定了二乙醇氨基甲酸钾盐的酰胺键的水解平衡常数,并建立了这类反应的平衡常数计算方法。     

Influence of the side chain on the stability of Schiff-bases formed between pyridoxal 5′-phosphate and amino acids

The stability of some Schiff-bases formed between PLP and different amino acids has been investigated in a wide range of pH. The kinetic constants of formation of these compounds and their hydrolysis rate constants have been determined. Results show that the α-position of the carboxyl group of amino acid plays an important role on the mechanism of water attack upon the C?N? bond. The absence of ionic groups in the surroundings of that bond must be an important factor of stability. Bulky hydrophobic substituents in the amino acid, near the amine part, protect the imine bond against hydrolysis.     

1,3-THIAZOLIDINIC AND 2,4,5,6-TETRAHYDRO-1,3-THIAZINIC SPIROCHROMENES AND MEROCYANINES. SYNTHESIS,REACTIVITY AND PHOTOCHROMIC PROPERTIES

Abstract

The acid catalyzed rate for hydrolysis of methylphosponfluoridic acid has been determined at several hydrogen ion concentrations and temperatures. The acid hydrolysis is second order (in acid and substrate). Assumed rate expressions, observed rate constants, and hydrogen ion concentrations were used to calculate the thermodynamic equilibrium constant (K _a=0.56) and rate constants for acid catalysis. The activation energy E _a has been determined as 18.3 Kcal/mole. Finally, the acid catalyzed deuterolysis was determined to be about 1.47 times the rate of hydrolysis. The data suggest a two-step mechanism consisting of a rapid proton transfer, followed by slow hydration of the protonated complex.     

Kinetics of the hydrolysis of woody-fibrous mass

The possibility has been shown of obtaining fodder additives by the hydrolysis of woody-fibrous mass with dilute sulfuric acid solutions. The kinetics of hydrolysis have been studied, a calculation has been made of the effective rate constants of hydrolysis of the polysaccharide part of the woody-fibrous mass, and the range of variation of the effective activation energy of the process has been determined.     

Kinetic study of N-bromosuccinimide reactions and kinetic determination of pyridoxine using a bromide-selective electrode

The bromide released by the alkaline hydrolysis of N-bromosuccinimide and its reactions with bromide, pyridoxine and thiamine have been studied potentiometrically using a solid-state bromide-selective electrode to monitor the consumed or produced bromide ion. Potential-time indications are obtained using a microcomputer-controlled potentiometric system. The overall rate constants and the activation energies have been calculated. A kinetic-potentiometric procedure for the determination of pyridoxine in the presence of thiamine in pharmaceutical preparations, based on its reaction with N-bromosuccinimide, is presented.     

Kinetics of hydrolysis of 4-methoxyphenyl-2,2-dichloroethanoate in binary water-cosolvent mixtures; the role of solvent activity and solute-solute interactions

Rate constants are reported for the pH-independent hydrolysis of 4-methoxyphenyl-2,2-dichloroethanoate in aqueous solution as a function of the concentration of added cyanomethane (acetonitrile), polyethylene glycol (PEG 400) and tetrahydrofuran (THF). The concentration of water was varied between ca. 25 and 55.5 M. It was found that the variation in water activity yields only a minor contribution to the observed variation in rate constants. Interestingly, for both cyanomethane and PEG 400 log(k) varies approximately linearly with the molar concentration of water. Medium effects in highly aqueous solutions ([H(2)O] > 50 M) of ethanol, 1-propanol, 2-propanol, 1-butanol and 2-methyl-2-propanol have also been determined. Unexpectedly, in this concentration range the alcohols induce significantly smaller effects per unit volume than cyanomethane. The present results are discussed in terms of pairwise interaction parameters. Isobaric activation parameters have been determined and reveal remarkable differences in the nature of the induced medium effects.     

Mechanism of alkane transfer-dehydrogenation catalyzed by a pincer-ligated iridium complex

The mechanism of (PCP)Ir-catalyzed transfer-dehydrogenation has been elucidated for the prototypical substrate/acceptor couple, COA/TBE, at 55 degrees C (COA = cyclooctane; TBE = tert-butylethylene). The catalytic cycle may be viewed as the sum of two reactions: (i) hydrogenation of TBE by (PCP)IrH2 and C-H addition of a second mole of TBE to give (PCP)IrH(tert-butylvinyl), and (ii) dehydrogenation of COA by (PCP)IrH(tert-butylvinyl) to give (PCP)IrH2, COE, and TBE. These two stoichiometric reactions have been observed independently and their kinetics determined. The overall catalysis has also been monitored in situ, and (PCP)IrH2 and (PCP)IrH(tert-butylvinyl) have been observed as the resting states; the ratio of these two complexes is found to be proportional to [TBE]2. Based upon the proportionality constant thus obtained and the catalytic rate as a function of [TBE] (which reaches a maximum at ca. 0.3 M), the respective rate constants for the hydrogenation and dehydrogenation segments can be obtained. Good agreement is found between the rates independently obtained from stoichiometric and catalytic runs. Within the overall TBE-hydrogenation reaction, labeling experiments indicate that the rate-determining step is the reductive elimination of TBA (2,2-dimethylbutane) from (PCP)IrH(tert-butylethyl) (which is formed via insertion of TBE into an Ir-H bond of (PCP)IrH2). Based upon considerations of microscopic reversibility, it can be further inferred that the rate-determining step for the alkane dehydrogenations is C-H addition (and not beta-H elimination).     

The Acid-Catalyzed Hydrolysis of Anomeric Alkyl Fructofuranosides

Abstract

The rate constants for the hydrolysis of some al-kyl α- and β-d-fructofuranosides in aqueous perchloric acid have been determined at various temperatures. The effects of varying the aglycon structure on the hydrolysis rate suggest, together with the markedly positive entropies of activation, that the substrate, protonated on the glycosidic oxygen atom, undergoes a rate-limiting unimolecular heterolysis to form a glycosyl oxocar-benium ion. The rate variations in mixtures of aqueous perchloric acid and dimethyl sulfoxide are interpreted as lending further support for the proposed mechanism.     

A simple capillary electrophoresis with electrochemical detection method for determination of the hydrolysis rate constant of chlorogenic acid

A method based on the kinetics stability study on hydrolysis of chlorogenic acid by capillary zone electrophoresis with electrochemical detection (CE-ED) has been developed in this paper. Both cyclic and hydrodynamic voltammograms of chlorogenic acid and its hydrolysis product caffeic acid have been investigated. The conditions for separation of chlorogenic acid and caffeic acid, such as the buffer pH and concentration, the separation voltage, and the injection time have been optimized. Under the optimum CE running conditions, the effects of reaction temperature and pH values of the hydrolysis solutions on the hydrolysis rate constants were further studied. The hydrolysis rate constants of chlorogenic acid were obtained from the concentration change of hydrolysis during the process of hydrolysis. Based on the fact, a simple and economical method for the determination of the hydrolysis rate constant and activation energy of hydrolysis reaction has been developed.     

Kinetics of the reaction of tris(4,7-diphenyl-1,10-phenanthroline-disulphonate)iron(II) ion with hydroxide ion in water

Summary The kinetics of the hydrolysis of the tris(4,7-diphenyl-1,10-phenanthrolinedisulphonate)iron(II) ion has been studied. The results are consistent with a consecutive reaction mechanism with a reversible step, and from the analysis of the kinetic results the rate constants have been determined.     

Stereoselective hydrolysis of amino acid esters by dipeptide catalysts and by N-decanoyl-histidine in surfactant aggregate domains

The rate constants of hydrolysis of the enantiomers of amino acid nitrophenyl esters by catalytic domains composed of cationic surfactant aggregates and dipeptide catalysts or Ndecanoyl-L -histidine have been determined at pH 7.30. The dipeptide catalysts shows large rate enhancement and stereoselectivity in aggregate domains. The surfactant structural effects are examined by investigation of the rate constants and stereoselectivities, and the nature of stereoselective catalysis is discussed.     

Kinetic study on the alkaline hydrolysis of 1-aryl-2-phenyl-2-imidazolines. Basicity-rate of hydrolysis and structure relationships

Rate constants (k_obs) of hydrolysis in boiling alkaline ethanolic solution for six 1-aryl-2-phenyl-2-imidazolines were determined. The influence of substituents in the phenyl group at N-1 upon rate of hydrolysis was studied. When the imidazoline ring is considered to be a substituent of the benzene ring at N-1, a good correlation with the Hammett equation is found. It was observed that reaction rates were enhanced by electron-releasing phenyl substituents of N-1 and reduced by electron-withdrawing groups, providing a change in the mechanism of the reaction in the first case that was not observed in the second. Agreement with the Hammett equation allowed comparison between experimental and “calculated” rate constants which are nearly equal. An equation relating the rate constants with the ionization constants of imidazolinium ions is given.     

Hydrolysis of oligosaccharides by polyelectrolytes

The hydrolysis of maltotriose, maltose, cellobiose and gentiobiose, at a concentration of 1% (w/v), have been investigated in 0.20 N sulphuric acid and 0.2 N polystyrene sulphonic acid. The apparent rate constants determined by high pressure liquid chromatography are compared and their dependence on temperature is given. The thermodynamic parameters of reaction with both acids are deduced. The catalytic effect of the polyacid, expressed by the ratio of the rate constants at a given temperature, is discussed in terms of the condensation of the counterions.     

First kinetic determination of partition coefficients for organic compounds between the three microenvironments of AOT-based microemulsions.

Kinetic data for the hydrolysis of N-picolinoylimidazole (I) and 2,4-dinitrophenylpicolinate (II) in AOT [bis(2-ethylhexyl) sodium sulfosuccinate] microemulsions are used to determine for the first time the two partition constants for each substrate (i.e. K(wi) and K(oi), corresponding to the incorporation of substrate molecules from water microdroplets and the continuous medium, respectively, into the interface). Application of the pseudophase formalism to the partition constants allowed the rate constant in each phase to be determined. The rate of hydrolysis of II increased with decreasing polarity of the medium; as a result, the hydrolysis reaction took place largely at the interface. On the other hand, the rate of hydrolysis of acylimidazole I decreased with decreasing polarity, possibly as a result of changes in the resonance structures of the reagent causing the hydrolysis process to occur preferentially in water microdroplets.     

HPCE analysis of hydrolysing morphine derivatives. Quantitation of decomposition rate and mobility

Summary High performance capillary electrophoretic conditions were optimised on the basis of protonation constants and hydrolysis rate constants (if relevant) of 6 opiate compounds. Protonation constants were determined by pH-potentiometry, the progress of hydrolysis was followed by capillary electrophoresis, hydrolysis rate constants of ester-type compounds were elucidated by kinetic studies of the time- and pH-dependence of the decomposition. Using these physico-chemical parameters, the analysis circumstances were designed to keep thein situ hydrolysis rate negligible for every compound, which has not been the case in reported previous HPCE determinations of acetylated derivatives. Our calculated charge-related mobility differences and experimental CZE findings justified those earlier statements that capillary zone electrophoresis is insufficient to separate these compounds. The method development for diacetylmorphine, 3-acetylmorphine, 6-acetylmorphine, morphine, acetylcodeine and codeine resulted in the use of a micellar elecktrokinetic system operating at pH=8.0, applying 50 mM sodium dodecyl sulfate micelle-forming agent and 7.5% acetonitrile additive in the background electrolyte.